This invention relates to electric motors in general, and specifically to laminated rotors for induction motors.
The use of laminated rotors for induction motors is well known. See, for example, U.S. Pat. No. 5,767,607, the disclosure of which is herein incorporated by reference. Typically, such a laminated rotor is comprised of a shaft with a center axis and a set of axially-mounted laminations constructed of a ferro-magnetic material.
The laminations of these types of rotors are quite versatile in that they may be sized and configured with punch patterns. Prior art laminations have been punched or stamped with circular shaft holes. Prior art laminations have also been punched with polygonal center holes that are then further machined into circular shaft holes.
Advantages of the laminations, however, are somewhat offset by the difficulties encountered in mounting them to a rotor shaft. Typically, the laminations are provided with a circular shaft hole centered in the lamination disks. A stack of laminations is applied to a circular rotor shaft by a cold press process. The force-fit cold press process presents several challenges to assembling the rotors.
First, the interference between the shaft and the shaft hole is typically about 0.0005 inches. It has been found that this is the largest interference, and hence produces the tightest fit, that permits a cold press application to the shaft. A larger interference will prevent the shaft from being pressed into the shaft hole of a stack of laminations. To achieve this precise fit, the inner diameter of the laminations and/or the outer diameter of the shaft require time consuming and relatively expensive finishing. Consistent machining then becomes a premium, as deviations from optimum finishing causes excessive press forces and mispresses.
Second, the tolerance of the shaft hole in the laminations is typically about 0.0007 inches. These close tolerances also present a challenge in press fitting the laminations onto the shaft so that the laminations are "straight" or perpendicular relative to the shaft. This is typically ensured via employment of a straightness plug gage which is 0.0002 inches below the minimum diameter of the shaft hole. If the laminations are not sufficiently straight relative to the shaft, they will not pass the gage. The assemblies that fail to pass the gage are scrapped.
Third and finally, the success of a cold press fit is critically dependent on the relative temperatures of the rotor shaft and the laminations that the shaft is pressed into. Because of the relative coefficients of thermal expansion between the two materials, variations in temperature between the shaft and the rotor may result in excessively tight or excessively loose fits, both of which have undesirable consequences. Whether the fit is too tight or too loose, the integrity of the rotor may be compromised, the performance of the motor may fluctuate, and the probability of failure of the rotor may be increased.
Therefore, the laminations of the prior art are disadvantaged in that they result in considerable scrap in production of the rotors, produce a wide variety of torque retention of the rotor to the shaft, and consequently produce variation in the performance of the rotor.